Yoast or All-in-One SEO Plugin
Cbnet Ping Optimizer
Google Analyticator
The one I use the most actively is SEOPressor
Google XML sitemaps
Webmaster tool verification

Oh yeah I have a site ranking for a local search term at #3 - semi competitive term. Was able to get it there in 4 weeks. The plug ins definitely helped - but the main thing was getting the on-page SEO right as a foundation and then doing off-page SEO. No plug in will get you around those foundational activities.

I use All in One SEO but a WP Plugin is not what put me on Page One...

Platinum SEO or Yoast. Personally, I find Platinum easier to use but either will give good results.

SEO

1. All in One SEO Pack
2. SEO Slugs
3. Google XML Sitemaps
4. SEOPressor
5. WordPress Ping Optimizer
6. WP No Category Base

Helpful Plugins

1. Spam Free Wordpress
2. Transposh Translation Filter
3. WSA Favicon
4. Robots Meta
5. Pinoy Pop-up on Exit
6. Daily Stat

To your success

-Lee

Personally, I like Yoast, but IMO theres very little difference between All-in-One SEO, Ultimate SEO, Headspace, Platinum SEO and so on. I also have several sites doing well without any SEO plugins at all.

I always try to use as few plugins as possible. Plugins generally slow your site down and can cause conflicts. Using too many creates a heavy bloated site which will negatively affect both SEO and user experience.

I use only several plug-ins such as
all in one seo
yoast
akismet
social media plugin
and yarp

yoast for the win

"SEO Ultimate" has a lot of features and hasn't cost me a dime.

i use all in one seo plugin in my wordpress.